Currently, in order to lower the resistivity of N-type silicon single crystals, the silicon (Si) is mixed with a dopant such as metallic arsenic (As), antimony (Sb) or phosphorus (P). Also, concurrently, the oxygen concentration of the silicon single crystal is sought to be controlled to a prescribed value in order to perform intrinsic gettering (hereinbelow called IG) of the wafer.
As shown for example in FIG. 5, the chief constituent elements of a CZ device are: a crucible 2 arranged in a chamber 1; a heater 6 provided at the periphery of the crucible 2; a heat insulating body 7 provided at the outer periphery of the heater 6; a heat shield 8 provided along the inside face of the heat insulating body 7; and a melt surface flow-straightening member 12 arranged in the vicinity of the melt surface 2d of the silicon melt 2a. 
The crucible 2 comprises two members: for the portion in which the silicon melt 2a is stored, a quartz (SiO2) crucible 3 is employed, and a graphite (C) crucible 4 is employed for holding and keeping hot the quartz crucible 3, covering the bottom and side face of the quartz crucible 3.
The inside end P of the flow-straightening member 12 is arranged having a prescribed distance from the melt surface 2d while its other end Q is arranged so as to contact the upper edge of the heat shield 8.
A single crystal 2b can be manufactured by melting silicon raw material in the quartz crucible 3 and pulling a crystal upwards in the Figure while the crystal is grown, from a seed crystal, not shown, using a pulling mechanism 2e, from the melt surface 2d. 
However, in the manufacture of silicon single crystals, the quartz crucible 3 and the Si of the silicon melt 2a undergo an interface reaction, generating SiO; this SiO mixes with the silicon melt 2a. This SiO is highly volatile and easily evaporates from the melt surface 2d. 
This SiO condenses and adheres on the inner wall 1c of the chamber 1, the surface of the single crystal 2b, and components in the vicinity of the crucible 2, which are at comparatively low temperature; this adhering SiO again exfoliates therefrom, and may become mixed as foreign matter with the silicon melt 2a. Such foreign matter results in the crystal growing from the melt surface 2d becoming polycrystalline and, as a result, the yield of silicon single crystals is lowered.
Accordingly, in order to raise the yield of silicon single crystals, a melt surface flow-straightening member 12 is provided in the vicinity of above the melt surface 2d, so as to direct inert purge gas introduced from a purge gas introduction port 1d provided in a cylinder 1a by the flow-straightening member 12: the SiO evaporated from the melt surface 2d is thereby entrained in the purge gas and is then discharged to outside the chamber 1 from a discharge port 1e. The flow of purge gas is indicated in the Figure by the arrows G1 to G4.
Additionally, it is known that the above-mentioned dopants bond with oxygen, present in trace amounts in the silicon melt 2a, and evaporate as volatile oxides from the melt surface 2d. For example, when silicon single crystals doped with Sb are pulled there is considerable evaporation of the volatile dopant oxide Sb2O3 from the melt surface 2d. 
In particular, if a melt surface flow-straightening member 12 is present, when seeking to obtain silicon single crystals of low resistivity by doping the silicon raw material with Sb, evaporation of Sb2O3 is further promoted by the flow-directing action and flow speed effect of the purge gas, so silicon single crystals of the desired low resistivity cannot be obtained with high controllability.
Furthermore, as described above, it is necessary to control the oxygen concentration of the silicon single crystals to a prescribed value in order to raise the IG effect of the wafer, but evaporation of SiO and dopant oxides is promoted by the use of the melt surface flow-straightening member 12 with the result that the oxygen concentration in the silicon single crystal cannot be controlled in high concentration.
Accordingly, Patent Reference 1 discloses a technique wherein, by suitable modification of the shape of the flow-straightening member, the flow rate of purge gas in the vicinity of the melt surface is reduced as far as possible, thereby suppressing evaporation of dopant oxide and preventing lowering of the oxygen concentration of the silicon single crystal.
Also, Patent Reference 2 discloses techniques for preventing the oxygen concentration in the silicon single crystal from being lowered, by raising the pressure in the chamber and suppressing evaporation of dopant oxide, and for discharging the SiO to the outside of the chamber by employing a high-speed gas purge current as curtain gas in order to prevent condensation and adhesion of the SiO, which is evaporated and diffused from the melt surface, on the inner walls of the chamber.
Patent Reference 1    Japanese Patent Application Laid-Open No. 07-232994.
Patent Reference 2    Japanese Patent Application Laid-Open No. 10-182289.